The present invention relates generally to frequency correction in a mobile communication terminal and, more particularly, to the estimation of the frequency offset between a base station and mobile terminal in a mobile communication network.
In mobile telecommunication systems, there is typically frequency offset between the transmitter and the receiver. The frequency offset can be caused by oscillator mismatch in the transmitter and the receiver and/or Doppler shift. Under certain channel conditions, the frequency offset due to Doppler shift can be quite large, i.e. greater than 1 kHz. One scenario where large frequency offsets are expected is the high speed train (HST) scenario as defined in the Third Generation Partnership Project (3GPP) specification TS 36.104 where the user is traveling on a high speed train. Due to large Doppler shift in the HST scenario, the frequency offset can be ±2 kHz for the Evolved Universal Terrestrial Radio Access (E-UTRA) operating band 7 frequency. For the mobile communication systems to work properly, a frequency offset of this magnitude must be estimated and corrected.
Simple and effective algorithms exist to estimate the frequency offset. One common approach estimates the frequency offset based on the phase differences of reference symbols (or pilot symbols). The frequency offset foffset is estimated as the phase difference ΔΦ divided by the time interval Δt of the pilot symbols, and may be given by:
                              f          offset                =                                            Δ              ⁢                                                          ⁢              Φ                                      2              ⁢              π              ×              Δ              ⁢                                                          ⁢              t                                .                                    Eq        .                                  ⁢                  (          1          )                    Because the observable phase difference is limited to an absolute value less than π, the time interval of the pilot symbols determines the maximum frequency offset range that can be estimated. For the Physical Uplink Shared Channel (PUSCH) in LTE systems, the time interval of the reference symbols is 0.5 ms. The maximum frequency offset that can be estimated from PUSCH reference symbols is ±1000 Hz. For the Physical Uplink Control Channel (PUCCH) using format 2/2a/2b, the time interval for the reference symbols is approximately 4/14 ms. The maximum frequency offset that can be estimated from PUCCH reference symbols is thus ±1750 Hz. If the frequency offset is beyond these ranges, the phase differences of the reference symbols will “wrap-around” π or −π. The “wrap-around” frequencies corresponding to the phase differences of ±π are ±1000 Hz for PUSCH and ±1750 Hz for PUCCH.
The wrap-around effect creates ambiguity in estimating the frequency offset. For example, a frequency offset of −1750 Hz for the PUSCH will cause phase rotation and end up at point A in the complex plane as shown in FIG. 1. Similarly, a frequency offset of +250 Hz will cause a phase rotation of
  π  4and end up at the same point in the complex plane. In this example, the −1750 Hz and +250 Hz frequency offsets are indistinguishable. The potential for large frequency offsets beyond the resolvable range thus creates an ambiguity that needs to be resolved to determine the correct frequency offset.
A method for increasing the resolvable range of frequency offsets is described in the patent application WO 2010/060732 “Frequency Offset Estimation” combining two frequency offset estimates on the same received signal. In this disclosure, the two estimates are calculated from pairs of received symbols with different time difference between the first and the last received symbol. However, for each PUSCH and PUCCH format 2/2a/2b there is only one time difference between the reference symbols located in the same part of the spectrum so it is not possible to resolve the ambiguity from a single received signal. Furthermore, the PUSCH and PUCCH cannot be scheduled in the same subframe for the same user and one channel (PUSCH for example) may be scheduled more frequently than the other (PUCCH for example). Thus, there will be instances where there are no fresh raw estimates from both channels. In these cases, a channel has to resolve the ambiguity with its own single raw estimate.
Therefore, new techniques are needed for extending the resolvable range of frequency offsets.